Another one: Black SabbathDio used to do some Ozzy songs back in the day on the BS tours. Paranoid, NIB, War Pigs, and others.

I don't think we'd ever hear Ozzy doing Heaven & Hell or Children of the Sea.

2 reasons as pointed out by the Captain:

1. The singers are just too different. Vocal ability comes into play.2. Ozzy doesn't need to do any Dio songs. But Dio always had to cover the Ozzy era. Not only because he could, but because fans wanted to hear those tunes.